This invention relates to a balanced phase modulator used in the microwave band (1 to 100 GHz).
Conventionally, a phase modulator for microwave band of 1 to 100 GHz has a circuit configuration as shown in FIG. 4, for example. In this figure, a modulation signal terminal 19 is provided by connecting diodes 20 and 20' to a branch line 3 dB hybrid 21 equipped with an input terminal 22 and an output terminal 23. In this phase modulator, a microwave signal entering from the input terminal 22 enters the branch line 3 dB hybrid 21 to be subjected to modulation provided by the diodes 20 and 20'. Ordinarily, Schottky barrier diodes are used for the diodes 20 and 20'. Such conventional phase modulator is disclosed in Japanese Laid-open Patent Application No. 57-155865 (published Sep. 27, 1982).
FIG. 5 is a diagram showing the modulation characteristic of the phase modulator of FIG. 4 in terms of the relation between the modulation voltage Vm and the output signal voltage Vout. In this phase modulator, if it is used in the range of small value of the modulation voltage Vm, then it operates as a linear modulator according to the modulation characteristic.
In the conventional phase modulator shown in FIG. 4, therefore, there is a drawback that a signal loss becomes significant because it must be used in the range where the modulation voltage is small. In addition, the maximum input power for the modulator is a low value of +10 dBm because of the use of the Schottky barrier diodes for the diodes in the circuit, and its output power becomes of the order of -5 dBm. Accordingly, it is necessary to increase the number of stages of amplification in order to obtain a desired output power. However, such an increase in the number of amplification stages brings about a complication of the circuit configuration, and high cost. Moreover, there is a problem in that the output power level is too low and it is difficult to maintain a circuit impedance as a whole at a constant level. The Japanese Laid-open Patent Application No. 57-155865 also shows another circuit configuration which includes an input terminal 24, an output terminal 25, a coplanar converter 26, a slot line 27, DC cutting capacitors 28, PIN diodes 29 and 29', a modulation signal-and-fixed bias input terminal 30 and a fixed bias input terminal 31. In this figure, one PIN diode 29 receives a modulation signal and a fixed bias voltage and the other diode 29' receives a fixed bias voltage, as shown FIG. 6.
According to the circuit configuration of FIG. 6, a high output power level can be obtained by applying the PIN diodes. However, modulation linearity is not so desirable since respective characteristics of the PIN diodes are varied while operating. Further, a C/N ratio of a modulation output is low due to impedance change of the PIN diodes.